Their equipment isn't clean
A dirty espresso machine says a lot about the establishment and just as much about the barista. That steam wand should be getting wiped down before and after each drink, and if there's grime all over the machine, you're not in a place with much pride.
They look like they're from the distant past
That vest and handlebar moustache might mean they're focusing much more on their looks than their coffee.
They take forever to make an espresso shot
For most machines the sweet spot is 20-30 seconds. Some fancier machines can be up to a minute, but if it feels like your shot is taking a long time, that's a bad sign.
They treat the espresso machine like garbage
Flushing the espresso machine only takes a quick push of a button. Whether the barista clears the water before pulling your shot of espresso is an easy way to tell if they actually care about one of the more important tools of the trade.